This chapter reviews the use of protoplasts in plant virology. Isolated protoplasts have been described as "naked" plant cells because the cell wall has been experimentally removed by either a mechanical or an enzymatic digestion. They provide a unique system for clarifying the interactions between a plant virus and its host at the cellular level. Protoplasts can be isolated from a variety of plant tissues and cultured cells. Leaf mesophyll cells have been used most frequently for protoplast isolation. Protoplasts are usually purified following enzymatic digestion, by a combination of filtration, centrifuging, and washing. Infection of protoplasts by plant viruses was achieved by treating freshly isolated protoplasts with a virus or viral RNA inoculum in a buffered mannitol solution in the presence of a high-molecular weight polycation. Hybridization analysis is a new method that has been applied to the detection of plant viruses and viroids. The use of protoplasts derived from different plant species for studying geminiviruses has been investigated.